1. Field of the Invention
This invention relates to a hydrostatic drive system with at least one user that is connected to a pump. To control the user, there is a control valve device that can be actuated by means of an actuation signal, and a delivery flow regulation device that can be triggered by a load pressure signal of the user. The drive system has a safety device by means of which the triggering of the control valve device can be controlled with the actuation signal and the triggering of the delivery flow regulation device can be controlled with the load pressure signal. The safety device has a function position in which the actuation signal is routed to the control valve device and the load pressure signal is routed to the delivery flow regulation device, and has a safety position in which the control valve device is discharged by the actuation signal and the delivery flow regulation device is discharged by the load pressure signal.
2. Technical Considerations
Drive systems of the above type are used in work machines, such as industrial trucks, as hydraulic work systems with a lifting drive, a tilting drive, and at least one accessory user, such as a side loader, for example. The safety device is provided so that in the event of a disruption, a malfunction, or a defect in the drive system, the operation of the user can be prevented and hazardous operating conditions can thus be avoided.
A drive system is described in DE 102 24 731 A1. In that case, a safety device realized in the form of a safety valve is provided which has a safety position and a function position. In the safety position, the delivery flow regulation device is discharged by the load pressure signal and the control valves are discharged by the actuator signal. In the function position, the load pressure signal is routed to the delivery flow regulation device and the actuator signal to the control valves. In the event of a disruption, a malfunction, or a defect of the drive system, the operation of the user can be reliably prevented by a corresponding actuation of the safety valve into the safety position.
In a generic drive system equipped with a safety device of this type, however, it has been found that accessory users that are connected to the corresponding control valve by means of hydraulic couplings, such as quick-release couplings, for example, cannot be coupled and uncoupled easily.
When the safety valve is in the safety position, the pressure in the lines that connect the accessory users to the safety valve device is reduced and, therefore, the pressure at the user connections of the accessory users is reduced only by means of leakage at the pistons of the control valve devices. However, this leakage at the pistons of the control valve devices depends on the manufacturing tolerances of the valves and the temperature. When the control valve devices are manufactured with a high degree of precision and thus low tolerances and when the temperature is low, the pressure reduction at the user connections of the accessory users can be insufficient so that the quick-release couplings cannot be handled easily and/or with a reasonable application of force to disconnect or connect the connections of the accessory users with the corresponding safety valve device for the uncoupling or coupling of the accessory users.
When the safety valve is moved into the function position, although one side of the user can be discharged to the reservoir by an actuation of the control valve device when there is a dual-action accessory user, when the control valve device is actuated the other side of the user is connected to the delivery line of the pump. As a result of the load pressure of this user, which is transported to the delivery flow regulation device, a delivery pressure that corresponds to the load pressure of the user is immediately generated at the corresponding user connection. This means that the uncoupling of the quick-release coupling that corresponds to this user line cannot be done with the application of a reasonable amount of force. Nor can the accessory user be connected easily because, when the control valve device is actuated, even though one side of the user can be discharged to the reservoir, the other side of the user is in turn connected with the delivery line of the pump by means of the actuated control valve device. When the quick-release coupling is closed and the user is uncoupled, the delivery pressure of the pump immediately increases to a value specified by a safety device, for example a pressure governor valve or a pressure cutoff valve, that responds at a maximum delivery pressure. As a result of which, the quick-release coupling that corresponds to this user side cannot be actuated with the application of a reasonable amount of force to couple the accessory user.
Therefore, it is an object of this invention to provide a drive system of the general type described above but which makes possible improved handling and facilitates the coupling and uncoupling of the users.